Conventionally, detecting devices for detecting a plurality of target objects, dividing the plurality of detected target objects into a plurality of classes, and deriving the number (frequency) of the plurality of detected target objects per class are known. For example, JP2005-249398A discloses a measuring fish finder that creates a histogram (frequency distribution) by dividing a plurality of single fishes in a predetermined area into classes of body lengths (refer to FIG. 5A and paragraph [0038]). With the measuring fish finder, as described in the paragraph [0038], by selecting an arbitrary area on the display screen, the frequency distribution of the plurality of single fishes in the area can be created.
“Simrad ES70 Split beam fish finder” (produced by SIMRAD) also discloses that the frequency distribution of a plurality of single fishes in an arbitrary area on the display screen can be created by selecting the area, similarly to JP2005-249398A.
Incidentally, it is considered that the frequency distribution displayed on the detecting devices disclosed in JP2005-249398A and “Simrad ES70 Split beam fish finder” is created based on information regarding a plurality of single fishes stored in a memory. The information includes information regarding pings of the respective detected single fishes (positions in the horizontal axis of FIG. 5A in JP2005-249398A), depth positions of the detected single fishes (positions in the vertical axis of FIG. 5A), and echo intensities of the detected single fishes. In other words, with the detecting devices disclosed in JP2005-249398A and “Simrad ES70 Split beam fish finder,” lots of information described above must be stored in the memory so as to create the frequency distribution, and thus, a large capacity memory is required.